1. Field of the Invention
The present invention relates to a radio communication terminal and a gain control circuit to be applied to the radio communication terminal.
2. Description of the Related Art
Modulation is performed with use of amplitude variation elements in a modulation system for an up channel of a recent digital mobile communication system such as a π/4 shifts QPSK applied to a PDC (Personal Digital Cellular) system that is a mobile radio system being in current use, an OQPSK applied to a cdmaOne system, a W-CDMA system that is a mobile radio system planned to be used in the near future, a HPSK applied to a cdma 2000 system, etc. Therefore, there has been a problem in that, a saturation operation cannot be performed in a transmission power amplifier (will be referred to as a “PA”, hereinafter) of a mobile communication terminal that is a radio communication terminal applied to each of the above systems, and thus electric current consumption increases. Specifically, in a case where the mobile communication terminal comes close to a base station and lowers terminal transmission power, most of the power that is supplied from a battery to the PA is spent in vain.
To solve the above problem, in a case where the transmission power is not large, it has been considered to control and restrict power supply to the PA in correspondence with a transmission signal level of the mobile communication terminal. In general, when the power supply to the PA is restricted too much, linearity necessary for the relevant signal level is impaired, thereby adjacent channel leak power deteriorates. Therefore, if unevenness in manufacturing PAs or variation in linearity owing to changes in the ambient temperature is taken into consideration, it has been difficult to control the power supply. In addition, it is a common practice to prepare some margins for a way of restricting power supply to the PA in order to avoid such a risk. However, for this reason, it has not been achieved to lower power consumption sufficiently.
As a method to solve this problem, the inventor of the present application has exhibited in the U.S. patent application Ser. No. 09/934675 that an adjacent channel leak power ratio is monitored and, with use of the monitored information, the power supply of the PA can be restricted to the level at which an appropriate adjacent channel leak power ratio is provided.
However, in a case where the power supply to the PA is restricted, a problem arises in that a gain of the PA deteriorates in some ways of the restriction. FIG. 5 shows an example of a PA with a function of adjusting power supply. In FIG. 5, an input signal is supplied to a primary stage amplifier, which comprises a transistor Q1 and load inductor L1, from an input terminal through an input impedance matching circuit M1. Then the input signal is amplified through the primary amplifier. After that, the amplified signal is supplied to an output stage amplifier, which comprises a transistor Q2 and load inductor L2 through an inter-stage impedance matching circuit M2. Then the supplied signal is amplified in the output stage amplifier and outputted to an output terminal through an output matching circuit M3.
Power consumption of the PA is expressed by the product of the sum of collector current flowing through each of the transistors multiplied by power voltage Vcc. The collector current flowing through each of the transistors can be controlled through a bias control circuit 401 that comprises a transistor Q3 and resistance R1. By lowering applied voltage to a bias control terminal shown in FIG. 5, the collector current of the transistor Q3 is reduced. By doing this, the collector current of the transistors Q1 and Q2, which are connected to each other in a current mirror manner through choke coils LC1 and LC2, can be reduced.
However, when the collector current of the transistors Q1 and Q2 is reduced, a gain at an amplification stage of each transistor deteriorates. FIG. 6 shows an example of variation in the gain of the PA against the voltage applied to the bias control terminal. It shows a case where the lower the bias control voltage deteriorates, the less gain diminishes. As described above, in the case where the power supply to the PA is restricted with the result that the gain deteriorates, a problem arises in that the level of the transmission signal deteriorates lower than the level of up transmission signal that is primarily required by the base station.
As described above, when the adjacent channel leak power ratio is monitored so as to restrict the power supply of the PA with use of the monitored information to the level at which the appropriate adjacent channel leak power is provided, the gain of the PA deteriorates. Therefore, a problem arises in that the level of the transmission signal deteriorates lower than the level of up transmission signal that is primarily required by the base station.